1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device which charges and discharges gate input capacitance of an insulated gate transistor and a method of driving the transistor.
2. Description of the Background Art
FIG. 14 is a view showing a configuration of a semiconductor device in a first background art. As shown in FIG. 14, the semiconductor device of the first background art comprises an IGBT (Insulated Gate Bipolar Transistor) 7 which is an insulated gate transistor, a protection diode 8 working against a reverse voltage applied between an emitter and a collector of the IGBT 7, a gate driving circuit 3, a control power supply 15a which outputs a voltage of +15 V, a control power supply 15b which outputs a voltage of xe2x88x9215 V, a transistor 4a having a collector connected to the control power supply 15a, a transistor 4b having an emitter connected to the control power supply 15b and a resistor 5. Gate input capacitance 6 is parasitic capacitance generated structurally between a gate and the emitter of the IGBT 7.
In the semiconductor device of FIG. 14, in order to turn the IGBT 7 on, first, the gate driving circuit 3 turns the transistor 4a on and turns the transistor 4b off. When the transistor 4a is turned on, the control power supply 15a supplies the gate of the IGBT 7 with the voltage of +15 V. At this time, since the gate input capacitance 6 is present between the gate and emitter of the IGBT 7, electric charges are supplied by the control power supply 15a to the gate input capacitance 6, thereby producing a gate current. As the gate input capacitance 6 is being charged, a gate voltage of the IGBT 7 increases and the IGBT 7 is turned on when the gate voltage of the IGBT 7 reaches a threshold voltage or more. After that, the charge of the gate input capacitance 6 of the IGBT 7 is completed, the gate voltage becomes about +15 V and the gate current almost stops flowing therein. Further, in some cases, the gate current flowing during a period from the start to end of the charge of the gate input capacitance 6 is simply referred to as xe2x80x9ccharging currentxe2x80x9d.
When the IGBT 7 is turned off, the gate driving circuit 3 turns the transistor 4a off and turns the transistor 4b on. When the transistor 4b is turned on, the control power supply 15b supplies the gate of the IGBT 7 with the voltage of xe2x88x9215 V. At this time, since the electric charges are accumulated in the gate input capacitance 6 of the IGBT 7, the electric charges are extracted by the control power supply 15b, thereby producing the gate current in a direction reverse to that of the case where the IGBT 7 is turned on. As the gate input capacitance 6 is being discharged, the gate voltage of the IGBT 7 decreases and the IGBT 7 is turned off when the gate voltage of the IGBT 7 becomes less than the threshold voltage. After that, the discharge of the gate input capacitance 6 of the IGBT 7 is completed, the gate voltage becomes about xe2x88x9215 V and the gate current almost stops flowing therein. Further, in some cases, the gate current flowing during a period from the start to end of the discharge of the gate input capacitance 6 is simply referred to as xe2x80x9cdischarging currentxe2x80x9d.
FIG. 15 is a view showing a relation of the gate voltage Vge, the gate current Ige and a collector current IC at the time when the IGBT 7 of FIG. 14 is turned on, and the gate current Ige corresponds to the charging current of the IGBT 7. As shown in FIG. 15, when the transistor 4a is turned on, the gate current Ige for charging the gate input capacitance 6 largely flows, in other words, the charging current largely flows, and after that, as the gate input capacitance 6 is being charged, in other words, as the gate voltage Vge increases, the gate current Ige decreases and when the gate voltage Vge becomes almost equivalent to the voltage outputted from the control power supply 15a, the gate current Ige almost stops flowing.
As discussed above, with the charging current supplied by the control power supply 15a, the IGBT 7 is turned on. In other words, the control power supply 15a needs power capacity to supply the charging current. Similarly, in order to turn the IGBT 7 off, the control power supply 15b needs power capacity to supply the discharging current. Then, in order to increase a rated current between the emitter and collector of the IGBT 7, it is usually necessary to increase the chip size of the IGBT 7, which leads to an increase of the gate input capacitance 6. For this reason, driving the IGBT 7 having a large rated current needs the control power supplies 15a and 15b having large power capacity. Further, when the IGBT 7 is used for an inverter device, as the operating frequency of the inverter device, i.e., the switching frequency of the IGBT 7 becomes higher, the charging current flowing per unit time becomes larger. For this reason, faster driving the IGBT 7 needs the control power supplies 15a and 15b having large power capacity. Thus, as the rated current of the IGBT 7 increases, and as faster driving of the IGBT 7 is desired, the power capacity of the control power supplies 15a and 15b required to drive the IGBT 7 increases.
The increase in power capacity of the control power supplies 15a and 15b as discussed above leads to an increase in cost and packaging volume of the control power supplies 15a and 15b. For this reason, in recent years when it is desired to reduce the cost and size of semiconductor devices, reduction of required power capacity of the control power supplies 15a and 15b is needed.
Then, a second background art is proposed, where the required power capacity of the control power supplies 15a and 15b are reduced. FIG. 16 is a view showing a configuration of a semiconductor device in the second background art. The semiconductor device of the second background art further comprises capacitors 11a and 11b besides the configuration of the first background art discussed above.
As shown in FIG. 16, when both the transistors 4a and 4b are in an off state, the capacitors 11a and 11b are charged by the control power supplies 15a and 15b. Then, in order to turn on the IGBT 7, when the gate driving circuit 3 turns the transistor 4a on and turns the transistor 4b off, electric charges accumulated in the capacitor 11a go through the transistor 4a to be supplied to the gate input capacitance 6 of the IGBT 7 where the supplied electric charges are accumulated. By the way, the gate current Ige of the IGBT 7 largely flows first when the transistor 4a is turned on and after that gradually decreases, as shown in FIG. 15. In short, the control power supply 15a of the first background art needs current supplying capability to produce the peak value of the gate current Ige as shown in FIG. 15. In the above-discussed second background art, since the electric charges accumulated in the capacitor 11a are supplied to the gate input capacitance 6 when the transistor 4a is turned on, the current supplied directly to the gate input capacitance 6 by the control power supply 15a decreases. For this reason, the control power supply 15a of the second background art does not need the current supplying capability to produce the peak value of the gate current Ige as shown in FIG. 15. In other words, it is possible to reduce the required power capacity of the control power supply 15a. 
Further, in order to turn off the IGBT 7, when the gate driving circuit 3 turns the transistor 4a off and turns the transistor 4b on, the electric charges accumulated in the gate input capacitance 6 go through the transistor 4b to be supplied to the capacitor 11b. For this reason, like for the control power supply 15a, it is possible to reduce the required power capacity of the control power supply 15b. Furthermore, almost the same technique as the second background art shown in FIG. 16 is disclosed in, for example, Japanese Patent Application Laid-Open No. 2-7617.
In the second background art, however, since the capacitors 11a and 11b are charged by the control power supplies 15a and 15b, respectively, the total amount of electric charges supplied by the control power supplies 15a and 15b is not reduced when the IGBT 7 is turned on or off.
Then, a third background art is proposed, where energy accumulated in the gate input capacitance 6 of the IGBT 7 is effectively utilized to ensure power savings of a semiconductor device on the whole. FIG. 17 is a view showing a configuration of a semiconductor device in the third background art. As shown in FIG. 17, the semiconductor device of the third background art comprises a MOSFET 17 which is an insulated gate transistor, a pulse signal source 24 for applying a pulse signal to a gate of the MOSFET 17, diodes 18 to 20, an inductor 21, a load 23 and a capacitor 22 for providing energy to the load 23.
In the third background art shown in FIG. 17, the pulse signal source 24 applies a pulse signal to the gate of the MOSFET 17 through the diode 20, thereby switching the MOSFET 17. When the MOSFET 17 is in an off state, the gate input capacitance 6 of the IGBT 7 and the inductor 21 resonate, to move energy which is accumulated in the gate input capacitance 6 when the MOSFET 17 is in an on state to the capacitor 22 through the diode 19. Then, the energy accumulated in the capacitor 22 is supplied to the load 23. Thus, in the third background art, since the energy accumulated in the gate input capacitance 6 is reused, loss of the energy in the semiconductor device on the whole can be reduced to consequently ensure power savings of the semiconductor device on the whole. Further, almost the same technique as the third background art shown in FIG. 17 is disclosed in, for example, Japanese Patent Application Laid-Open No. 8-163862.
The third background art, however, is not a technique to reduce the required power capacity of a power supply to drive the MOSFET 17, like the control power supplies 15a and 15b, e.g., a power supply for the pulse signal source 24 and reduce the size of the power supply. Further, in order to quickly move the energy of the gate input capacitance 6 to the capacitor 22, it is necessary to resonate the gate input capacitance 6 and the inductor 21 and in order for that, it is necessary that the signals supplied from the pulse signal source 24 to the gate of the MOSFET 17 should have constant frequency and duty. For this reason, the semiconductor device shown in FIG. 17 can not operate the MOSFET 17 with an arbitrary switching frequency.
It is an object of the present invention to provide a semiconductor device and a method of driving a transistor, in which electric charges accumulated in gate input capacitance of an insulated gate transistor are effectively utilized to reduce required power capacity of a power supply to drive the transistor and ensure power savings of the semiconductor device on the whole.
According to a first aspect of the present invention, the semiconductor device includes an insulated gate transistor, a driving unit and a capacitor.
The driving unit produces charge and discharge of gate input capacitance of the insulated gate transistor, and the capacitor is linked to the charge and the discharge produced by the driving unit to be selectively connected to a gate of the insulated gate transistor.
The capacitor is linked to the discharge produced by the driving unit to be connected to the gate, whereby the gate input capacitance supplies the capacitor with electric charges accumulated therein and the capacitor accumulates therein the electric charges supplied from the gate input capacitance.
The capacitor is linked to the charge produced by the driving unit to be connected to the gate, whereby the capacitor supplies the gate input capacitance with the electric charges accumulated therein and the gate input capacitance accumulates therein the electric charges supplied from the capacitor.
In the semiconductor device according to the first aspect, since the capacitor is linked to the discharge of the gate input capacitance provided by the driving unit to be connected to the gate of the insulated gate transistor, when the capacitor is connected to the gate of the transistor in advance of the discharge of the gate input capacitance produced by the driving unit, for example, the gate input capacitance supplies the capacitor with the electric charges accumulated therein and the capacitor accumulates therein the electric charges supplied from the gate input capacitance in advance of the charge of the gate input capacitance produced by the driving unit. Further, since the capacitor is linked to the charge of the gate input capacitance produced by the driving unit to be connected to the gate of the insulated gate transistor, when the capacitor is connected to the gate of the transistor in advance of the charge of the gate input capacitance produced by the driving unit, for example, the capacitor supplies the gate input capacitance with the electric charges accumulated therein and the gate input capacitance accumulates therein the electric charges supplied from the capacitor. As a result, when being charged by the driving unit, the gate input capacitance of the insulated gate transistor eventually accumulates the electric charges which are accumulated therein at the time of the discharge of the gate input capacitance produced by the driving unit. Therefore, since the electric charges are accumulated in the gate input capacitance to some degree when the driving unit begins charging the gate input capacitance, it is possible to reduce the amount of electric charges to be supplied to the gate input capacitance by the driving unit until the charge of the gate input capacitance is completed. As a result, it is possible to reduce the required power capacity of the power supply to allow the driving unit to supply the gate input capacitance with the electric charges.
Furthermore, since part of the electric charges which are accumulated in the gate input capacitance of the insulated gate transistor at the time of the discharge thereof are used to charge the gate input capacitance, in other words, the electric charges accumulated in the gate input capacitance are effectively used, it is possible to ensure power savings of the semiconductor device.
According to a second aspect of the present invention, the method of driving the transistor includes the steps of (a) and (b). In the step (a) discharge of gate input capacitance in an insulated gate transistor is produced, and in the step (b) charge of the gate input capacitance is produced.
The step (a) includes the steps of (c) and (d). In the step (c) electric charges accumulated in the gate input capacitance are supplied to the capacitor and the electric charges supplied by the gate input capacitance are accumulated in the capacitor, and in the step (d) electric charges remaining in the gate input capacitance are extracted after the step (c).
The step (b) includes the steps of (e) and (f). In the step (e) electric charges accumulated in the capacitor are supplied to the gate input capacitance and the electric charges supplied by the capacitor are accumulated in the gate input capacitance after the step (d), and in the step (f) further electric charges are supplied to the gate input capacitance after the step (e).
In the method of driving a transistor according to the second aspect, when being charged by the driving unit, the gate input capacitance of the insulated gate transistor eventually accumulates the electric charges which are accumulated therein before the discharge of the gate input capacitance. Therefore, since the electric charges are accumulated in the gate input capacitance to some degree when the step (f) is executed, it is possible to reduce the amount of electric charges to be supplied to the gate input capacitance until the charge of the gate input capacitance is completed. As a result, it is possible to reduce the required power capacity of the power supply to supply the gate input capacitance with the electric charges in the step (f).
Further, since part of the electric charges which are accumulated in the gate input capacitance of the insulated gate transistor at the time of the discharge thereof are used to charge the gate input capacitance, it is possible to effectively use the electric charges accumulated in the gate input capacitance.